Accuracy of amount of laser radiation can be improved by closely controlling on/off periods of light emitting devices, such as laser diodes, for use in laser printers or the like. It is possible to improve accuracy of radiation time and amount of radiation as periods, during which the light emitting devices are turned on and off, can be controlled more closely during a predetermined period. More specifically, when the control is performed using pulse width modulation (PWM), the accuracy of radiation time and amount of radiation can be improved as combination of various widths increases.
Generally, a pulse width modulation (PWM) pulse signal is generated in the following manner. As is the case of a PWM pulse signal generation device 100 employing a counter shown in FIG. 1, a base clock supplied from a base clock oscillator 10 is counted by a counter circuit 20. In a selector circuit 30 to which a preset counted value is input, if the counted value reaches a specified value set by PWM level setting, the selector circuit inverts ON/OFF (1/0) of output of the selector circuit. A PWM pulse waveform shaping circuit 40 generates pulse waveforms by combinations of ON/OFF. Although this method is a simple method, the number of combinations of various pulse widths is limited during one cycle specified by the base clock frequency, which makes it difficult to increase resolution. For example, in a case where a 10-bit pulse width modulation pulse waveform is generated, the base clock frequency is limited to approximately 200 MHz and the basic period is about 0.2 MHz from 200 MHz/1024. This is insufficient for a frequency required in practical application of laser printers or the like.
Due to a configuration that a base clock is counted by a counter, a constraint that the operation upper limit frequency of a counter composed of a plurality of logic circuits cannot be increased further is imposed on a PWM pulse signal generation device employing a counter circuit. Since a counter having more number of digits has to be produced to increase the resolution further, the logic circuits become more complex and the size of the logic circuits on a semiconductor circuit board becomes larger, which prevents the further increase in the frequency.
As means for increasing the frequency, there are PWM pulse signal generation devices employing a ring oscillator in which a plurality of inverters are connected in serial to create a ring and that oscillates. This is a method utilizing delay of logic circuits. The oscillation frequency is determined by a signal delay speed of delay elements, which are inverters. Since the frequency depends on the temperature, the voltage, etc., and accuracy of pulse width modulation is not obtained without any measures, ring oscillators are generally controlled by a PLL (phase lock loop) so that variation in the frequency is not caused.
FIG. 2(a) shows an example of a configuration of a PWM pulse signal generation device 200 employing a ring oscillator. A plurality of delay elements 211 to 214 are connected in serial. An output terminal of a last delay element is connected to an input terminal of a first delay element through an inverter (an inverting element) 216. A ring oscillator 220 is formed so that a desired base frequency can be divided equally utilizing signal propagation delay caused by the delay elements (D) 211 to 214. A width of an output pulse of each of the delay elements 211 to 214 is controlled by a control voltage V1 applied to each delay element. An oscillation frequency of the ring oscillator 220 is determined by a delay time of a delay element (D). Since the delay time is affected by disturbance, the ring oscillator 220 operates as a VCO (voltage controlled oscillator). A PLL (phase lock loop) is formed by a PLL control circuit 230 constituted by a phase comparator 232, a low-pass filter (e.g., integrator) 234, and a voltage control circuit 236 and stabilizes the oscillation frequency by locking the signal output from the ring oscillator to an external clock pulse input from an external oscillation circuit 240. The pulse signal output from each of the delay elements of the ring oscillator 220 is input to a selector circuit 250. After the selector circuit 250 selects either a non-inverted pulse or an inverted pulse, a pulse waveform shaping circuit 260 generates PWM pulse waveform with various pulse widths. FIG. 2(b) shows an example of a waveform of the output pulse from each delay element and PWM output pulses having performed pulse width modulation (PWM) in a case where the number of the delay elements is four. Pulses output from the delay elements (D) 211 to 214 at the first cycle are denoted as DO (t0), D1 (t1), D2 (t2), D3 (t3), and D4 (t4). Pulses output at the second cycle are denoted as D0*, D1* and so on. As shown in the figure, at the second cycle, the pulse waveform is inverted. As examples of PWM output pulses, a PWM output 1 is obtained by operation of D1-D3 (subtraction) and a PWM output 2 is obtained by operation of D2-D1* (subtraction).
In this method, even when a ring oscillator with 512 stages is used, a basic period (a value obtained by dividing the base frequency by resolution) is limited to approximately 15 MHz. This method is generally used as a high-speed PWM pulse signal generation system currently. The basic period is shortened by lowering the resolution. That is, it is difficult to generate a pulse with a pulse width corresponding to a basic period not greater than a delay time that a delay element has, thus limiting the resolution.
A pulse width modulation (PWM) circuit capable of generating output signals having a uniform pulse width with respect to a given pixel clock and of being formed on one chip is disclosed in Japanese Unexamined Patent Application Publication No. 4-151968. In this patent document, a pulse width modulation circuit including (1) a voltage controlled oscillator VCO composed of a plurality of ring oscillators, (2) a tap selection circuit for selecting one signal having a waveform closest to that of a detect signal from output signals of each delay element of the VCO and for outputting an image clock, (3) a delay circuit including delay elements that are the same as those of the VCO, to which the control signal of the VCO being supplied similarly, and for outputting delay signals delayed by a predetermined time with respect to the pixel clock, (4) a waveform shaping circuit for forming pulse signals having pulse widths according to the outputs from this delay circuit, and (5) a waveform selection circuit for selecting an output signal corresponding to input image data from the output signals from the waveform shaping circuit, is disclosed. For example, when a clock having a phase shifted by T/8 is input to the waveform shape circuit, clock signals PWM1 to PWM7 having pulse widths of ⅛T, ¼T, . . . , and ⅞T are generated, respectively. That is, waveforms whose pulse widths are shifted by a constant time with respect to a given frequency T can be obtained. It is mentioned that multilevel image data is input to the waveform selecting circuit, whereby the corresponding PWM signal is selected. However, in this PWM circuit, an amount of delay cannot be set to a value not greater than an amount of delay (a delay time) that a delay element has, which does not lead to increase of the resolution.